Particularly in telecommunications applications and data technology applications, overvoltage protection plugs are used to protect the wired-up double wires against overvoltages and overcurrents caused by technical defects or external disturbances such as lightning strikes. To this end, such plugs have a component which limits the voltage and, generally, the current as well and to which a fuse element, generally in the form of a solder pellet, is assigned. If an overcurrent lasting for a relatively long time occurs in the voltage-limiting component, then the solder pellet melts as a result of the heat losses occurring on the component, as a result of which a mechanical fail-safe device generally shortcircuits the wires to ground. This is necessary since the overvoltages or overcurrents that occur could possibly adversely affect the serviceability of the components. In order to make it easier to identify which fail-safe device has tripped, various visual fail-safe signaling means are already known, lightemitting diodes being used predominantly.
Various solutions are known for signaling the tripping of a fail-safe device. When the voltage-limiting component overheats, it is also possible, in addition to the short-circuit from a and b to ground, for a parallel process to be initiated (indirect signaling). The safety-relevant process of fail-safe tripping in this case remains entirely uninfluenced by the signaling process, but the fact that the relationship is only approximate is disadvantageous. Even if the two processes are optimally matched, thermal constellations are always possible in which this fail-safe device has tripped but signaling does not take place, or vice versa.
Solutions for direct signaling means are known, in which, once a solder pellet has melted, the moving part of the fail-safe device makes contact, by means of spring force on reaching its limit position, not only with the contact points for the wires a and b but also with a further contact point for signaling. The design must in this case be configured such that the contact point for signaling is designed to have weak springing in order to avoid impeding the fail-safe movement. In any case, it is necessary to avoid an excessively low contact force occurring, or even no contact being made whatsoever, by the third contact point on the two contact points a-ground and b-ground.
It is known for the components for the overvoltage protection plug to be arranged on a printed circuit board. Since the visual fail-safe signaling is not always desirable or necessary, two types of printed circuit boards are therefore manufactured, namely one with and one without visual fail-safe signaling. The additional components for visual fail-safe signaling also result in the geometric dimensions of the printed circuit board being increased. However, in order to allow the same housing to be used for both printed circuit board versions, this is designed to match the larger printed circuit board. Furthermore, when the plug is arranged in a housing, this housing must be designed such that the visual fail-safe signaling can be perceived. To this end, the housing has an opening, out of which the light-emitting diode arranged at the plug end can project so that it can be perceived visually. This opening must then be closed f or the plug version without visual fail-safe signaling.